I think costly signaling is the wrong phrase here. Costly signaling is about gain for the signaler. This seems better modeled as people trying to indirectly purchase the good "rich people donate lots to charity.". Similar to people who are unwilling to donate to the government (so they don't think the government is better at spending money than they are) but do advocate for higher taxes (meaning they think the government is better at spending money than other people are). They're trying to purchase the good "higher taxes for everyone".

Maybe, but the thing I'm trying to get at here is "a bunch of people saying that rich people should donate to X" is a less credible signal than "a bunch of people saying X thing is important enough that they are willing to donate to it themselves."